NERVOUS IRRITABILITY. 365 



within outward, transmitting a special nerve force, adapted to excite 

 muscular contraction. 



It is evident, however, that these reasons do not indicate a real differ- 

 ence in the activity of the nerve fibres, but only in the sensible results 

 of its operation. In neither rase is there any perceptible effect produced 

 in the nerve, but only in the organ with which it is connected. When 

 a sensitive nerve is excited, the sensation is perceived in the nervous 

 centre ; when a motor nerve is called into activity, contraction takes 

 place in the muscle. It is possible that the condition of the nerve 

 under excitement may be the same in both cases, and that the differ- 

 ence in effect may be due only to the organ in which it terminates : 

 just as the conducting wire of a galvanic battery may be made to ring 

 a bell or move an index, according to the mechanism with which it is 

 connected. There are some facts which can hardly bear any other 

 interpretation than this, and which lead to the conclusion that the 

 physiological action in the two kinds of nerve fibres is not essentially 

 different. 



1. The stimulus applied to a nerre. either .^n^ifire or motor, pro- 

 the same effect throughout its entire length. 



Impressions made upon the integument, which give rise to sensation, 

 arc transmitted by the sensitive nerve through its whole course to the 

 nervous centre ; and the sensation thus produced is referred, not to the 

 brain or to any part of the nerve trunk, but to its point of distribution 

 in the integument. An irritation applied to the same nerve in the mid- 

 dle of its course produces a sensation which still seems to come from 

 the integument. After the amputation of a limb in man, if the severed 

 extremity of a nerve be compressed or irritated in the cicatrix, the 

 sensations excited are referred to the amputated limb ; and patients 

 often assert that they can feel the separated parts as distinctly as 

 before. The impression conveyed through the remaining portion of 

 the nerve is the same as if the whole of it were still in existence. 



The motor nerves act in a similar way. A voluntary stimulus orig- 

 inating in the brain passes through the entire length of a motor nerve 

 to reach the muscles and excite their contraction. If the nerve be 

 divided at any intermediate point, and a galvanic stimulus applied to 

 the peripheral portion, contraction follows in the muscles as before. In 

 each case, the physiological effect is produced at the extremity of the 

 nerve fibres ; and is apparently of the same character, from whatever 

 distance it has been transmitted. 



It appears accordingly that the nerve fibre, whether sensitive or 

 motor, when excited, is thrown into a condition of activity throughout 

 its length; the nerve assuming a state of "polarity," analogous to 

 that of a magnetized bar, in which the visible phenomena of attrac- 

 tion or repulsion are manifested only at its extremities, although the 

 intermediate portions of the bar participate in its molecular action. 

 When the exciting stimulus, in a sensitive nerve, is applied at the 

 peripheral extremity, it must necessarily be transmitted from without 



